I don't think she said whether it was a desk-to-pig or pig-to-desk transformation, she'd told people not to guess and she asked Harry to wait until after class about his speculation.

But she should have also warned against using free Transfiguration to convert something into an animal. It's kind of covered under not transforming things into liquids which can evaporate but you'd expect her to be explicit.

She might also have pointed out that guessing whether it was a pig-to-desk or desk-to-pig free transformation would have got the wrong answer either way. Sure it's a trick question, but so was asking whether Harry cared to guess in the first place.